Mark no-fake-mousemove.html as Slow

When running with VizDisplayCompositor virtual/user-activation-v2/fast/events/no-fake-mousemove.html
was flaking with timeouts. However it was only flaking on one particular bot
WebKit Mac10.13 (retina).

We suspect that the extra process is leading to increased delay in this test.

Marking this test as slow will allow us to confirm that.

TEST=virtual/user-activation-v2/fast/events/no-fake-mousemove.html

Bug: 912364
Change-Id: Ic04f50efa612782fdf66ecc4cb2ea260defb04a1
Reviewed-on: https://chromium-review.googlesource.com/c/1372470Reviewed-by: Ria Jiang <riajiang@chromium.org>
Commit-Queue: Jonathan Ross <jonross@chromium.org>
Cr-Commit-Position: refs/heads/master@{#615758}

third_party/blink/web_tests/SlowTests

@@ -1573,3 +1573,5 @@ crbug.com/893015 [ Linux ] virtual/streaming-preload/http/tests/fetch/chromium/r
 crbug.com/893015 [ Linux ] virtual/streaming-preload/http/tests/fetch/chromium/response-form-data-gc-crash.html [ Slow ]
 crbug.com/893015 [ Linux ] virtual/streaming-preload/http/tests/fetch/chromium/response-json-gc-crash.html [ Slow ]
 crbug.com/893015 [ Linux ] virtual/streaming-preload/http/tests/fetch/chromium/response-text-gc-crash.html [ Slow ]
+
+crbug.com/912364 [ Mac ] virtual/user-activation-v2/fast/events/no-fake-mousemove.html [ Slow ]